As described in co-owned European Patent No. EP1813695 (De Bosscher et al) and co-pending U.S. application Ser. No. 13/179,825 (“the '825 Application”), the entire disclosures of which are incorporated by reference, rotary sputtering targets are used for coating large surface areas of substrates with thin films of various materials, by bombarding the target in a closed inert atmosphere chamber with negative ions to dislodge atoms of the target material for deposition onto the surface of a substrate positioned in and/or moved through the chamber at a controlled rate. If not in an enclosed chamber, the negative ions can be retained in a space between the target and the substrate by maintaining a magnetic field just above the target surface, by a process known as magnetron sputtering. Such rotary targets typically are tubular in shape and may have lengths up to 152 inches. Depending on the properties of the target material, a cylindrical backing tube may be positioned inside rotary target to support the target, and through which backing tube a cooling fluid (often water) is circulated during the sputtering process to prevent the target from becoming excessively heated.
Rotary thin film deposition targets, whether used in physical or chemical vapor deposition processes, have been shown to improve the deposition process. By rotating the target during the deposition process, the target material is applied in a more continuous uniform fashion to larger substrate surfaces than might planar sputtering targets.
If for example a material that is to be deposited by sputtering is soft or malleable and has a high weight to strength ratio, such as silver or gold, the rotary sputtering target as the material is being sputtered away becomes thinner and thus is more likely to flex or break during rotation. To alleviate this problem, such rotary sputtering targets typically are supported by a backing tube able to resist target flexing. The backing tube allows for more of the target material to be sputtered without deformation of the target and therefore enables higher yields when compared to systems lacking a backing tube.
Backing tubes usually are formed from materials that have a lower cost than the target material and that can withstand the deposition process while retaining its shape. Generally, backing tube materials should have high thermal conductive properties, which is especially important for rotary target materials having low melting points. Backing tube materials having a low thermal conductivity could result in a thermal gain that would lead to an incipient melting situation, resulting in an electrical or electrostatic short in the deposition process, dangerous arcing within the system, premature target failure and damage to the substrate. Backing tubes should also have the rigidity and strength to support the target material, a liquid cooling fluid such as water, and a magnet array internal to the tube to minimize bending of the assembly when supported on one or both ends.
A rotary deposition target and backing tube assembly can be made by casting or spraying the deposition material onto the backing tube, if the sputtering material is castable or sprayable. Casting or spraying will have a number of drawbacks with specific substrates, and can be detrimental to deposition systems, as they often result in variable grain sizes, leading to less consistent deposition, and will have an inherent porosity that is created from the volume changes in the liquid to solid transition during casting or spraying.
The rotary deposition target and backing tube assembly may include, for example, a sleeve positioned between the target and backing plate substantially along the full length of the target, which sleeve may be either thermally conductive or thermally reflective depending on the material to be deposited and the deposition equipment used. However, during the deposition process, the heat that is generated at the outer surface of the target during deposition is then transferred into the bulk of the target through to the inner surfaces of the target. If the thermal expansion of the target relative to the backing tube causes the target and/or sleeve to lose physical contact with the backing tube, much of the cooling effect achieved by the physical contact with the backing tube will be lost and the differential thermal expansion increased even more.
Although the descriptions that follow are for rotary sputtering target and backing tube assemblies and the sputtering process, it is to be understood that the embodiments as are illustrated and described can be used in analogous other plasma vapor deposition and treatment processes.
A prior art rotary sputtering target and backing tube assembly is made by use of a bonding material as illustrated schematically in FIG. 1, in which a pre-formed rotary sputtering target 10 is positioned over and fixed to a backing tube 12 by bonding material 11 extending in a continuous fashion along the entire length of the target from its first end 13 to its second end 14. The depth of bonding material 11 usually is negligible, in that only a minimum amount of bonding material is needed to affix target 10, but solely for illustrative purposes is exaggerated in FIG. 1. As the bonding material, an adhesive or bonding alloy such as indium or an indium-tin alloy is inserted between the inner surface of the rotary sputtering target 10 and the outer surface of the backing tube 12 to create a strong bond between the two surfaces. Care must be taken when pouring the adhesive between the sputtering target and the backing tube to ensure the minimal uniform spacing between them for the adequate bonding strength. Reuse of the backing tube and of remaining target material in pure form after the target has been used in the sputtering process may be compromised when separated from each other and from the bonding material.
An improved method for cooling the sputtering target during sputtering is to perforate the backing tube, as disclosed in the co-owned European Patent No. EP 1813695 to De Bosscher et al. As explained in that patent, perforations in the backing tube allow for partial direct contact of the cooling fluid with some of the target's inner surface and thus enhance somewhat the thermal transfer. If direct contact of the cooling fluid with the target is not desired, a thin membrane or sleeve that is leak proof and thermally conductive may be used between the target material and the backing tube, still enabling a higher thermal transfer than is possible if the backing tube were not perforated. Sleeves having vacuum and water sealing properties may be used both to support/fix the target in place and to prevent leakage of the cooling fluid from the interior of the backing tube. Having a low cost and high weight to strength ratio perforated backing tube that can enable a more direct cooling of a target is desired, provided that a reliable bond nevertheless is achieved.